Preceramic compositions and ceramic products

ABSTRACT

Preceramic polymer dispersions which have particular utility in providing protective ceramic coatings having low moisture sensitivity on carbon/carbon composites, graphite, carbon fibers, and other normally oxidizable materials are prepared by dispersing about 0-3 parts by weight of aluminum-silicon eutectic, about 0-4 parts by weight of silicon carbide, about 1.5-5 parts by weight of silicon boride, and about 0.4-5 parts by weight of silicon metal in a solution or dispesion obtained by dispersing about 0.1-1.0 parts by weight of a Group IIA metal salt in an organoborosilazane polymer obtained by reacting about 0.25-20 parts by weight of a trialkoxy-, triaryloxy-, or tri(arylalkoxy)boroxine with one part by weight of a polysilazane in an organic solvent and, if desired, heating the dispersion to convert it to a solution.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser.No. 373,574, filed June 30, 1989 and now abandoned.

FIELD OF INVENTION

This invention relates to ceramic materials derived from polysilazanesand more particularly to such materials which are useful in protectingsubstrates that are normally susceptible to oxidative deterioration.

BACKGROUND

It is known that many materials, such as carbon/carbon composites,carbon fibers, graphite, and certain metals, have properties which makethem attractive for use in aerospace and other applications in whichtheir susceptibility to oxidative deterioration at elevated temperaturesis a serious disadvantage. It would be desirable to find a means ofprotecting those materials from oxidation at high temperatures, and ithas been proposed to provide such protection with ceramic coatings.However, known ceramic coatings have proved to be inadequate.

Copending applications Ser. No. 242,493 (Niebylski-I), filed Sept. 9,1988, and Ser. No. 272,481 (Niebylski-II), filed Nov. 17, 1988, and nowU.S. Pat. No. 4,921,925 teach organoborosilazane polymers which can becoated onto substrates and pyrolyzed to ceramics to provide improvedprotection from oxidative deterioration at elevated temperatures; andcopending application Ser. No. 261,104 (Niebylski-III), filed Oct. 24,1988, teaches that coating compositions comprising theorganoborosilazane polymers can be improved by the incorporation thereinof a mixture of aluminum-silicon eutectic, silicon carbide, siliconboride, and silicon metal. However, the utility of these compositions islimited by the moisture sensitivity of the ceramics obtained from them.

THE INVENTION

It has now been found that oxidizable substrates can be protected fromoxidative deterioration at elevated temperatures with ceramics havinglow moisture sensitivity and derived from compositions obtained bydispersing (A) a homogenized mixture of about 0-3 parts by weight ofaluminum- silicon eutectic, about 0-4 parts by weight of siliconcarbide, about 1.5-5 parts by weight of silicon boride, and about 0.4-5parts by weight of silicon metal in (B) a solution or dispersionobtained by dispersing about 0.1-1.0 part by weight of a Group IIA metalsalt in an organoborosilazane polymer solution obtained by reactingabout 0.25-20 parts by weight of a trailkoxy-, triaryloxy-, ortri(arylalkoxy)boroxine with one part by weight of a polysilazane in anorganic solvent and, if desired, heating the dispersion to convert it toa solution.

The organoborosilazane polymers employed in preparing theorganometallosilazane polymers are those disclosed in Niebylski-I andNiebylski-II, i.e., polymers prepared by reacting about 0.25-20 parts byweight of a trialkoxy-, triaryloxy-, or tri(arylalkoxy)boroxine with onepart by weight of polysilazane.

The polysilazane which is reacted with the boroxine may be anypolysilazane that is soluble in common organic solvents, such asaliphatic or aromatic hydrocarbons, dialkyl or alicyclic ethers, etc.;and it may be, e.g., a polysilazane of any of U.S. Pat. Nos. 4,397,828(Seyferth et al.-I), 4,482,669 (Seyferth et al.-II), 4,645,807 (Seyferthet al.-III), 4,650,837 (Seyferth et al.-IV), and 4,659,850 (Arai etal.), the teachings of all of which are incorporated herein in toto byreference. However, it is preferably a polysilazane of the type tauqhtby Seyferth et al.-II, i.e., a polysilazane prepared by reactinq anorqanodihalosilane with ammonia, treating the ammonolysis product with abasic catalyst which is capable of deprotonating an NH group that isadjacent to an SiH group, and quenching the resultant product with anelectrophilic quenching reagent, a mixture of such polysilazanes, or,alternatively, an oligomeric ammonolysis product formed as anintermediate in the process of Seyferth et al.-II and isolated as inSeyferth et al.-I. For example, it may be one or more polysilazanesprepared by reacting methyldichlorosilane with ammonia, treating theammonolysis product with potassium hydride, and quenching the resultantproduct with methyl iodide or dimethylchlorosilane; or it may be one ormore polysilazanes prepared by reacting methyldichlorosilane withammonia and isolating the ammonolysis product.

The boroxine reactant used in preparing the organoborosilazane polymeris a compound corresponding to the formula: ##STR1## wherein R is analkoxy, aryloxy, or arylalkoxy group, preferably an alkoxy, phenoxy,alkylphenoxy, phenalkoxy, or alkylphenalkoxy group in which any alkyl oralkoxy group contains 1-6 carbons, such as the trimethoxy-, triethoxy-,tripropoxy-, tributoxy-, tripentoxy-, trihexoxy-, triphenoxy-,tritolyloxy-, tri(2-ethylphenoxy)-, tribenzyloxy-, triphenethoxy-,tri(3-phenylpropoxy)-, tri(4-phenylbutoxy)-, tri(5-phenylpentoxy)-, andtri(6-phenylhexoxy)boroxines, the corresponding triphenalkoxyboroxineshaving non-linear alkyl chains, tritolylethoxyboroxine, etc. It ispreferably trimethoxyboroxine or triphenoxyboroxine.

Regardless of the particular boroxine used, the amount employed is about0.25-20 parts per part by weight of the polysilazane. However, when theboroxine is a trialkoxyboroxine, it is generally preferred to use about1-6, most preferably about 3-4 parts per part by weight of polysilazane;and, when the boroxine is a triaryloxyboroxine, it is generallypreferred to employ about 1-10, most preferably about 6-8 parts per partby weight of polysilazane.

To prepare the organoborosilazane polymers, the neat boroxine reactant(if sufficiently low melting) or a solution thereof in an organicsolvent is added to a solution of a polysilazane in an organic solventto initiate an exothermic reaction which is preferably controlled to atemperature below 50.C. for a period of time sufficient to allow theformation of an organoborosilazane polymer. In a preferred embodiment ofthe invention, the polysilazane is used as a clear solution having asolids content of about 10-40%, preferably about 30% by weight; and thetotal amount of solvent employed is such as to provide anorganoborosilazane polymer solids content of about 5-75%, preferablyabout 30-60% by weight.

The solvent employed for the polysilazane and optionally also theboroxine may be any suitable organic solvent, such as hexane, heptane,and other aliphatic hydrocarbons; benzene, toluene, xylene, and otheraromatic hydrocarbons; cyclohexanone, 1-methyl-2-pyrrolidone, and otherketones; etc.; and mixtures thereof.

The Group IIA metal salt which is mixed with the organoborosilazanepolymer is a salt, such as a fluoride, tetrafluoroborate, oxide,oxyfluoride, oxynitride, acetate, benzoate, etc., of beryllium,magnesium, calcium, strontium, or barium. The calcium and barium saltsare preferred, with the fluorides and tetrafluoroborates thereof beingparticularly preferred.

As indicated above, the dispersions of the invention may be heated toconvert them to solutions if desired. It is believed that thermaltreatment of the dispersions causes the salt to react with theorganoborosilazane polymer, although it is possible that heating merelysolubilizes the salt. When solution formation is desired, it isgenerally accomplished by heatinq the dispersion at a temperature in therange of about 120-150° C. for a suitable time, e.g., about 12-24 hours.

The solids which are intimately mixed with the aforedescribeddispersions and solutions to form the dispersions of the invention areconstituted by about 0-3 parts by weight of aluminum-silicon eutectic,about 0-4 parts by weight of silicon carbide, about 1.5-5 parts byweight of silicon boride, and about 0.4-5 parts by weight of siliconmetal per part by weight of polysilazane employed in making theorganoborosilazane polymer. The silicon carbide is preferably α-siliconcarbide, the silicon boride may be silicon tetraboride and/or siliconhexaboride, and the silicon metal is preferably amorphous.

In the preparation of the dispersions, it is preferred to premix thesilicon boride, silicon metal, and any aluminum-silicon eutectic and/orsilicon carbide, homogenize and dry them, and then intimately mix themwith the Group IIA metal-organoborosilazane polymer solutions ordispersions. Generally, the Group IIA metal-organoborosilazane polymersolutions or dispersions are added to the homogenized solids, whetherpredispersed or not, and the resultant dispersions are agitated untilthey are uniform.

When the homogenized solids are predispersed in an organic medium, theamount of organic medium used is generally such that the ultimatedispersion has a total solids content of about 5-75% by weight,preferably about 30-60% by weight, if the dispersions are to be used ascoating and/or infiltration materials.

The dispersions of the invention are preceramic materials which areuseful for making ceramics, such as coatings, structural composites,etc.; and, like other preceramic materials, they may be used incombination with other ingredients, such as ceramic powders or whiskers,etc., when appropriate.

An application in which they find particular utility is as coatingcompositions for normally oxidizable materials, especially those whichneed protection from oxidative deterioration at elevated temperatures.(Such materials include, e.g., fibers, tows, hanks, mats, and compositesof carbon; carbon or graphite slabs, rods, and structures; andoxidizable metals, such as magnesium, aluminum, silicon, niobium,molybdenum, lanthanum, hafnium, tantalum, tungsten, titanium, and themetals of the lanthanide and actinide series.) When used in such anapplication in which the substrate is porous, the compositions alsoserve as infiltrants.

In addition to providing protection from oxidative deterioration, thecoating compositions can also serve to improve the physical propertiesand thermal stability of substrates, such as those mentioned above,silica foams, ceramic cloths (e.g., clothes formed from alumina, silica,and/or lithia), etc.

The coating compositions are also useful for patching ceramic coatingsformed from the same or different formulations.

When the dispersions are to be used to provide protective ceramiccoatings on substrates, the surfaces to be coated are usually cleanedprior to the application of the coating composition in order to improvethe bonding of the ceramic coating to the substrate. The bonding cansometimes be further improved by pre-etching the surfaces to be coated.

The coating compositions may be applied to the substrates in anysuitable manner, such as by spraying, swabbing, or brushing, to formcoatings having the desired thickness, generally a thickness of up toabout 1000 micrometers, frequently a thickness of about 10-250micrometers. A coating of a desired thickness can be achieved byapplying a single coating of that thickness or by applying the precursorcoating composition in multiple thinner layers, e.g., by applying thecoating composition in layers of about 25-100 micrometers, each layerbeing dried by driving off the solvent before the next layer is applied.

When temperatures as high as about 200-250° C. are used to drive offhigh boiling solvents, some pyrolysis of the preceramic material isinitiated during the drying of the coating composition. However, highertemperatures, i.e., about 675-900° C., preferably about 825-875° C., arerequired to convert the preceramic coating to a ceramic coating. Thispyrolysis may be delayed until the final desired thickness of preceramiccoating has been deposited. However, it is generally preferred topyrolyze each one or two layers of dried preceramic coating beforeapplying the next layer of coating composition. The time required forthe pyrolysis is generally about 1-60 minutes, depending on theparticular pyrolysis temperature selected. In the preferred embodimentof the invention where the coating is applied in multiple layers, eachone or two of which is pyrolyzed before the application of the nextlayer, and the pyrolysis temperature is about 825-875° C., it isgenerally preferred to pyrolyze the first coat for only about fiveminutes and then to pyrolyze subsequent coats for longer times up toabout 15 minutes.

When the coating is intended to protect a substrate from oxidativedeterioration at very high temperatures, e.g., temperatures higher than800° C., the final pyrolysis is followed by thermal treatment of thecoated substrate at about 1075-1250° C., preferably about 1100-1175° C.,most preferably about 1125° C., in an atmosphere containing not morethan a minor amount of oxygen, e.g., in a nitrogen, argon, or heliumatmosphere, to convert the ceramic coating into a homogeneous film. Thistreatment may be accomplished by raising the temperature in the vesselused for the pyrolysis or by transferring the coated substrate to avessel maintained at the higher temperature; and it is preferablycontinued for about five minutes for the first coat and longer periods,e.g., about 15-20 minutes, for subsequent coats.

After the pyrolysis or pyrolysis/heat treatment, the coated substrate iscooled. Optimum results are attained when this cooling is accomplishedat a rate not greater than about 50.C/minute, preferably about 20-30°C./minute, until the substrate temperature is below 500° C., at whichtime further cooling may be accomplished at ambient air temperature.

Although not essential, it is preferred to keep the startingpolysilazane and the organoborosilazane polymers and compositions formedfrom it in a dry atmosphere until a layer of ceramic has been formedbecause of the susceptibility of the preceramic materials to attack bywater and other compounds having active hydrogens.

As already indicated, the dispersions of the invention are useful inpreparing a variety of ceramic objects, but the major advantage of theinvention is its provision of compositions capable of resistinghydrolytic attack and protecting normally oxidizable materials fromoxidative deterioration at elevated temperatures. This advantage is ofparticular importance in the protection of carbon/carbon composites,graphite, and metals used in aerospace applications, such as enginecomponents, advanced nozzle system components, and high-temperaturevehicle structures.

The following examples are given to illustrate the invention and are notintended as a limitation thereof.

EXAMPLE I Synthesis of Polysilazane Part A

A suitable reaction vessel was charged with 14L of anhydroustetrahydrofuran and cooled to about 0.C, after which 1545g (13.43 mols)of methyldichlorosilane was added to the vessel, and stirring at about60 rpm was begun. A slow steady stream of 1058g (62.12 mols) ofanhydrous ammonia gas was introduced into the vessel at a flow rate suchthat the reaction pressure was maintained at or below 400 kPa, and thereaction temperature stayed in the range of 0-10° C. Then the reactionmixture was stirred at 0.C for about three hours, after which thecoolant flow on the vessel was shut off, and the system was put undergentle nitrogen purge to allow the reaction mass to warm to roomtemperature and the majority of the excess ammonia to vent off. Then thereaction vessel was pressurized with sufficient nitrogen gas to pump theproduct mass through a bag filter assembly into a holding tank, where itwas verified that the filtrate solution was free of particulates.

Part B

The clear filtrate from Part A was discharged into a polymerizationvessel and chilled to about 0° C., and a suspension of 3.6g (0.089 mol)of potassium hydride powder in about 100 mL of anhydrous tetrahydrofuranwas added to begin the polymerization reaction. The reaction mixture wasmaintained at 0° C. for about 8 hours and then allowed to warm graduallyto about 22° C. After a total of about 26 hours of polymerization at0-22.C, the reaction was quenched by adding about 12.6g (0.13 mol) ofdimethylchlorosilane to the polymerization solution.

The polymer product was isolated by (1) concentrating the productsolution to about 4L of volume by vacuum distillation, (2) centrifugingthe concentrated solution to obtain a clear supernatant solution and awhite precipitate, (3) decanting off the supernatant solution from theprecipitate, and (4) flashing off the volatiles from the supernatantsolution by vacuum distillation to provide a white solid. Proton NMRspectra of the polymer in deuterated chloroform solvent had resonancesconsistent with those reported in Seyferth et al.-II for polysilazaneand with the presence of a small amount, i.e., 2.4% by weight, ofresidual tetrahydrofuran.

EXAMPLE II Synthesis of Organoborosilazane Polymer

A clear solution of four parts by weight of trimethoxyboroxine in amixture of 0.5 part by weight of xylene and 0.5 part by weight of1-methyl-2-pyrrolidone was slowly added to a clear solution of one partby weight of the polysilazane of Example I in a mixture of 1.5 parts byweight of xylene and 1.5 parts by weight of 1-methyl-2-pyrrolidone. Anexothermic reaction occurred to form a solution of an organoborosilazanepolymer.

EXAMPLE III Synthesis of Group IIA Metal-Organoborosilazane PolymerSolution

Composition A, a solution, was prepared by dispersing 5g of a fineanhydrous barium fluoride powder in 100g of the organoborosilazanepolymer solution of Example II and stirring the composition continuouslywhile heating it overnight at 130.C to form a solution from whichundissolved salt (about 0.8g) was removed.

EXAMPLE IV Preparation of Dispersion

A mixture of 0.3 part by weight of aluminum-silicon eutectic, 1.7 partsby weight of o-silicon carbide, two parts by weight of siliconhexaboride, and 1.8 parts by weight of amorphous silicon metal washomogenized and vacuum-dried for at least two hours, after which theComposition A solution of Example III was added and intimately mixedwith it to form a dispersion designated as Composition B.

EXAMPLE IV

Graphite coupons having nominal dimensions of about 3.8 cm x 2.5 cm x0.3 cm were abraded to provided a smooth finish, cleaned, vacuum dried,thoroughly swab-coated in an inert atmosphere with Composition B, dried,heated at 100° C. for five minutes, heated to 150.C at a rate of about10.C/minute, held at 150° C. for 15-30 minutes, allowed to cool to roomtemperature, recoated and held at 150 C for 30 minutes, heated to about200-225° C., maintained at that temperature for at least 15 minutes, andcooled to provide coupons having a coating thickness of about 0 08-0.1mm.

The polymer coatings were then pyrolyzed to ceramic coats by heating thecoated coupons to 800-825° C., holding at that temperature for 30minutes, subsequently heatinq them at about 1125° C. for at least fiveminutes, and cooling to room temperature at a rate of 10-20° C./minute.

The effectiveness of the ceramic coats thus obtained in protecting thegraphite substrate from oxidation was determined by an oxidation test.The coated specimen was mounted horizontally in a half section of asilicon carbide tube which was used as a holder and which allowed over99% of the coupon surface to be directly exposed to hot ambientconvecting air. The holder and specimen were placed in a box furnacewhich had been preheated to 1100° C. Periodically the holder andspecimen were removed from the furnace and quenched in ambient air, thecooled specimen was weighed and remounted in its holder, and the holderand specimen were replaced in the heated furnace for additional heatingin air. The weight loss on oxidation was determined to be only 13% afterfour hours, compared with a weight loss of 67% after four hours whenuncoated graphite coupons were subjected to the same test and a weightloss of 56% when the graphite coupons subjected to the test were coatedwith Composition A instead of Composition B.

It is obvious that many variations may be made in the products andprocesses set forth above without departing from the spirit and scope ofthis invention.

What is claimed is:
 1. A dispersion of a homogenized mixture of about0-3 parts by weight of aluminum-silicon eutectic, about 0-4 parts byweight of silicon carbide, about 1.5-5 parts by weight of siliconboride, and about 0.4-5 parts by weight of silicon metal in a solutionor dispersion obtained by dispersing about 0.1-1.0 part by weight of aGroup IIA metal salt in an organoborosilazane polymer solution obtainedby reacting about 0.25-20 parts by weight of a trialkoxy-, triaryloxy-,or tri(arylalkoxy)boroxine with one part by weight of a polysilazane inan organic solvent and optionally heating the dispersion to convert itto a solution.
 2. The dispersion of claim 1 wherein the silicon carbideis α-silicon carbide.
 3. The dispersion of claim 1 wherein the siliconboride is silicon tetraboride.
 4. The dispersion of claim 1 wherein thesilicon boride is silicon hexaboride.
 5. The dispersion of claim 1wherein the silicon metal is amorphous.
 6. The dispersion of claim 1wherein the salt is a fluoride, tetrafluoroborate, oxide, oxyfluoride,or oxynitride.
 7. The dispersion of claim 6 wherein the salt is bariumfluoride.
 8. The dispersion of claim 1 wherein the boroxine istrimethoxyboroxine.
 9. The dispersion of claim 1 wherein thepolysilazane is a polymer prepared by reacting an organodihalosilanewith ammonia, treating the ammonolysis product with a basic catalystwhich is capable of deprotonating an NH group that is adjacent to an SiHgroup, and quenching the resultant product with an electrophilicquenching reagent.
 10. The dispersion of claim 9 wherein theorganodihalosilane is methyldichlorosilane and the basic catalyst ispotassium hydride.
 11. An article which comprises a substrate that isnormally susceptible to oxidative deterioration and a coating derivedfrom the dispersion of claim
 1. 12. A ceramic derived from thedispersion of claim 1.